The six-person advance team will be joined by two dozen more observers in coming days in line with a U.N. Security Council resolution adopted on Saturday authorizing the deployment of up to 30, Ahmad Fawzi said.

However, the Syrian government said on Sunday it had a right to refuse monitors depending on their nationalities. Government spokeswoman and presidential adviser Bouthaina Shaaban also said the government could not be responsible for the safety of the monitors unless it was involved in "all steps on the ground".

Four days after a ceasefire was meant to come into effect, Syrian government forces shelled the city of Homs on Sunday, resident opposition activists and a rights activist said.

"Of course we are hoping that the process holds together until the observers get on the ground," Fawzi told Reuters in Geneva.